Stabilized plastic chloroprene polymer and process of producing same



Patented Mar. 11, 1941 UNITED STATES PATENT OFFICE STABILIZED PLASTICCHLOROPRENE POLY- MER, AND PROCESS OF SALIE PRODUCING N0 Drawing.

Application December 14, 1938,

Serial No. 245,595 I 8 Claims. (Cl. 2150-89) This invention relates tothe treatment of plastic polymers of halogen-2-butadienes-1,3 and theirhomologues. to the treatment of plastic polymers of chloro-2-butadiene-1,3 (hereinafter, for convenience, referred to as chloroprene)More particularly, it relates to stabilization of the plastic polymersof these halogen dienes. This application is a continuation-in-part ofcopending application Serial No. 69,740, filed March 19, 1936.

Heat curable plastic polymers of chloroprene (that is, plastic polymersof chloroprene which are readily converted simply by the action of heatinto the strong elastic state resembling vulcanized rubber) andparticularly those made in emulsion in the presence of certain modifyingagents as disclosed, for example, in the copending applications ofStarkweather, filed March 19, 1936, and given Serial No. 69,739;Starkweather 8: Collins, filed March 19, 1936, and given Serial No.69,737; and Starkweather & Collins, filed March 19, 1936, and givenSerial No. 69,738, are soft and plastic when first isolated r and lendthemselves readily to the common operations of rubber technology such asmilling, calendering and tubing. On standing at ordinary temperatures orsometimes when allowed to warm during milling, these polymers have atendency to lose their plasticity and become tough and elastic, even inthe presence of antioxidants, such as phenyl beta naphthylamine. Thus,on storage under ordinary conditions they show a decided tendency tolose their plasticity within periods as short as three months and attemperatures of about 70 0. they exhibit this decided tendency within 24hours. It thus becomes very diflicult or impossible to handle them withordinary rubber machinery and hence their use is limited.

It is therefore an object of this'invention to maintain in the plasticcondition during milling and storage heat curable plastic polymers ofhalogen-2-butadienes-l,3 and their homologues in which the hydrogen atomon carbon atom 3 is replaced by a hydrocarbon group. A more specificobject is to maintain heat curableplastic polymers of chloroprene in theplastic condition during milling and on storage for periods much longerthan 3 months under ordinary conditions. A still further object is toprovide means for maintaining the plasticity of these polymers, whichmeans, however, does not substantially reduce their capacity to beconverted under curing conditions to the strong elastic Morespecifically it relates state resembling vulcanized rubber. Otherobjects will appear hereinafter.

With these objects in mind it has now been discovered that theplasticity of heat curable plastic polymers of halogen2-butadienes-1,3and their homo-logues of the general formula strong elastic stateresembling vulcanizedrubher, by having present with them relativelysmall amounts of certain compounds hereinafter referred to asstabilizers, such as aromatic mercaptans or'mercapto compoundscontaining aromatic groups or such as ammonium salts, the use of whichfor this purpose is specifically claimed in a copending application ofStarkweather and Collins, Serial No. 196,186, filed March 16, 1938. Ithas further been found that this invention is particularly applicable toheat curable chloroprene plastic polymers, especially those prepared asdescribed in the copending applications, identified above.

The following examples are included in order to more fully illustratethe invention, and it should be understood that these examples are givenfor this purpose alone, and that accordingly they are not to beconstrued as limiting the scope of the invention.

In these examples and throughout this application, whenever the termplasticity number? is used, it refers to the thickness in thousandths ofan inch of a 2.5 cc. sample of the polymer in the form of a cylinder indiameter which has been heated to 80 C. for 15 minutes and then keptunder a Weight of 5 kilograms. for three minutes at 80 C. It will beobvious that the plasticity will vary inversely as the plasticitynumber, lower plasticity numbers indicating more plastic materials. Ithas been found that to be milled satisfactorily a polymer should, ingeneral, have a plasticity number less than 125.

The term regain is also used in this application and wherever it occursherein refers to the gain in thickness in thousandths of an inch of thecompressed sample, from the plasticity test, when freed from pressurefor one minute at room temperature. For satisfactory milling this"regain should, in general, be less than 25.

throughout these examples, the term "parts is used, it refers to partsby weigh Example 1 A plastic polymer of chloroprene was preparedaccordingto the method disclosed in a copending application ofStarkweather and Collins,

Serial No. 69,738, filed March 19, 1936, by polymerizing chloroprene inacid aqueous emulsion in the presence of 0.75% of thioglycolic acid and0.06% of hydrogen sulfide (both based on the amount of chloropreneused).

The dried, finished polymer containing no stabilizer or antioxidant wasdivided into 2 portions, one of which was treated with 1% of phenyl betanaphthyla- 1 mine by incorporation on a rubber mill and was retained asa control.

The other portion was treated with 1% of phenyl beta naphthylamine and2% of thiophenol on a rubber mill. The

1 plasticity numbers and regain of both samples were determined at onceand after aging for 24 and 48 hours in an air oven, at 70 C. Thefollowing table gives the plasticity data thus obtained for polymertreated and untreated with thiophenol and also with several othersubstances coming within the scope of this invention. In each case 2% ofthe aromatic mercaptan is used.

Plasticity number and regain after aging at j Polymer Agent added inaddition to 70 C. for- No. phenyl beta-naphthyiamine Ohrs. 24hrs.-48hrs.

None(control) 71-2 121- 1am Thiophenoi 63-2 67-1 70-1 Thio-beta-uaphtho66-2 78-0 80-1 o-Thiocresol 65-1 77-2 79-2 None (control) 102-9 184-150202-178 'lhio-alpha-na hthol. 77-5 103-7 109-0 None (control 77-3 150-90p-Nitro thiophenol 69-1 99-3 102-6 None (control). 87-1 124-7 136-314...---. Benzylmercaptan 68-2 100-3 102-3 It will be seen frominspection of this table that all the mercaptans added have a verypronounced effect in reducing the change in plasticity on A similarimprovement over unobtain at ordinary temperature.

' Example 2 One hundred parts by weight of chloroprene were polymerizedin the presence of thioglycolic acid and hydrogen sulfide as describedin Example 1. When about 80% of the chloroprene had polymerized, adispersion of one part by weight of phenyl beta naphthylamine and 2parts by weight of para'nitro thiophenol dissolved in parts by weight ofbenzene and dispersed in parts by weight of a 1% solution of stearylsodium sulfate was added. The resulting mixed dispersion was treatedwith solid sodium chloride and the coagulated stabilized polymer waswashed and dried as described in the copending application mentionedinExample 1. The product had a stability similar to that of the stabilizedproduct of Example 1, and when 100 parts of this product were compoundedwith 5' parts of zinc oxide, 10 parts of magnesium oxide, and 5 parts ofrosin and cured at 130 0., a highly elastic rubber-like 75fproduct ofgood tensile strength wa obtained.

Other representatives oi this general class of compounds than thosegiven in the examples will be found to be operative. For example, we mayuse such compounds as:

m-Thiocresol p-Thiocresol o-Nitro thiophenol m-Nitro thiophenol Dinitrothiophenol;

Nitro thiocresols p-Mercapto diphenyl o-Mercapto diphenyl l-mercaptoanthracene 2-mercapto anthracene Chloro thiophenols Dichloro thiophenolsChloro nitro thiophenols Di-bromo thiophenols 'Iri chloro thiophenolsThiophenol sodium sulfonates Mercapto diphenyl potassium sulfonatesMercapto anthracene sulfonic acids Iodo thio cresols Di nitro thionaphthols Nitro benzyl mercaptans Chloro-b-phenyl ethyl mercaptans'I'hio xylenois 1 As illustrated by the above examples, the inventlonmay be applied to the stabilization in the plastic state of heat curableplastic polymers, generally of halogen-2-butadienes-1,3 and theirhomologues corresponding to the following general formula:

' in which X is halogen and R is hydrogen or a hydrocarbon radical; forexamples, bromo-2- butadiene-1,3 and chloro-2-methyl-3-butadiene- 1,3.When a mercaptan is incorporated into such polymers as'illustrated abovethe chloroprene polymers, it has been found that it exerts a definiteeffect by way of maintaining the plasticity of the polymer, 1. e.,"stabi1izing it as that term is used herein.

As is apparent from the above disclosure, many aromatic mercaptans areuseful in the present invention. The common functional part of thesecompounds is the SH group. The other substituents are used to transformcertain of the mercaptans that are slightly volatile and have strongobjectionable odors into practically nonvolatile compounds in which theodor is practically eliminated 'or greatiyreduced. For example,thiophenol imparts considerable odor to polymer treated with it, whereasthe para nitro thiophenol is practically odorless at room temperatureand gives no noticeable odor to'polymer in which it is used. Otherhydrocarbon substituents than methyl may also be present in the arylgroups or the mercapto group may bepresent in an aliphatic substituentof' the aromatic group.

.It is also withinthe scope of the present invention to add to thepolymers materials which under the conditions prevailing in the polymerto which they are added, will generate a mercaptan as described herein.

For the p ses of this invention, the arcmatic ring should not contain,however, any substituents such as the following which appear to havearr-adverse efi'ect: COOH, --NH, a second Y-SH, OH, or alkoxy group. Theresulting mercaptan compounds are either inactive or else theyaccelerate the change from the plastic to Ill the elastic conditionunder conditions of storage and milling.

Generally, therefore, it may be said that any mercaptan of the generalformula: Q-SH, in which the SH group is attached directly to one of thecarbon atoms in the Q group and in which Q is an organic group selectedfrom the class consisting of aryl groups and aralkyl groups, which arylgroups and aralkyl groups consist of carbon and inorganic radicalsselected from the class consisting of H, Cl, Br, -I,

NO2, and SOcM, in which M is an inorganic excess of stabilizing agentappears to remain as inert material. Preferably, therefore, about 1% to2%, based on the polymer, of the lower molecular weight stabilizingagents should be present. In the case of the stabilizing agents ofhigher molecular weight, larger proportions may be necessary to producesimilar effects.

For the purpose of this invention these stabilizing agents may be addedto the polymer at any time after the polymerization process has beencarried to the point desired, in amounts such that the total amount ofstabilizer present after the addition is, at least, that amount whichwill produce a definite stabilizing effect, i.- e., about 0.1% to aboutbased on the weight of the polymer, as disclosed above. It should alsobe understood that mixtures of two or more of the stabilizing agents,disclosed herein, may be used and that where two or more stabilizingagents are used their effects are, in general, additive, i. e., if oneis present and another is added, it will increase the stabilizingeffect. Itv is also possible to use a stabilizer of another class incombination with an aromatic mercaptan of the type disclosed herein. USuch a combination, using a substituted ammonium salt (the use of whichis broadly disclosed in a copending application of Starkweather andCollins, filed March 16, 1938, and given Serial No. 196,186) with anaromatic mercaptan is further discussed hereinafter. It will be obvious,of course, that two stabilizers either of which exerts a destructiveeffect on the other, can not be used together to advantage. This methodof stabilizing may be applied to any type of heat curable plasticpolymer of chloroprene. Thus, instead of the modified emulsion polymerof the type used above, a polymer, for example, made by the methoddescribed in U. S. Patent No. 1,950,436 may be used. Such a polymer, towhich 1% of phenyl beta naphthylamine only was added, changed inplasticity number and regain during two days in the accelerated agingtest at 70 from 77 and 3 to 117 and 10. Another portion, stabilized with1% of ortho thiocresol, in addition to the phenyl beta naphthylamine,changed from 68-4 to 94-6 under the same conditions of aging.

The invention is particularly applicable to the heat curable plasticpolymers produced by the process of the following copendingapplications: Starkweather, Serial No. 69,739, filed March 19,

1936, which discloses emulsion'polymerization of chloroprene in thepresence of sulfur dioxide as a modifying agent; Starkweather andCollins, Se-- rial N0. 69,737, filed March 19, 1936, which has nowmatured into U. S. Patent No. 2,163,250, granted June 20, 1939, whichdiscloses emulsion polymerization of chloroprene in the presence ofhydrogen sulfide as a modifying agent; and Starkweather and Collins,Serial No. 69,738, filed March 19, 1936, and refiled July 30, 1937 asS8- rial No. 156,518 which discloses emulsion polymerizationofchloroprene inthe presence of organic modifying agents. The organicmodifying agents which this latter case discloses are unpolymerizableacid-stable organic compounds which are capable of forming an additionproduct with compounds of the general formula in which X is hydrogen andR is a member of the group consisting of hydrogen and hydrocarbonradicals under conditions of polymerization of such compounds of saidgeneral formula. -This group of modifying agents includes the followingclasses of organic chemical compounds:

(1) Unsaturated compounds containing one or more carbonyl groupsadjacent to the double bond;

(3) Aromatic sulfinic acids;

(3) Aromatic mercaptans;

(4) Mercapto-carboxylic acids;

(5) Aliphatic mercaptans.

On standing at ordinary temperatures, or sometimes when allowed to warmduring milling, polymers made according to the above cases in thepresence of the stated modifying agents have a tendency to lose theirplasticity. The sulfur dioxide and hydrogen sulfide may be employedwhether or not the monomer is dispensed in nonalkaline or acid emulsion.It is still more particularly applicable to the heat curable plasticpolymers of chloroprene produced by polymerizing in acid aqueousemulsion according to the methods of these applications. Thestabilization of these latter plastic polymers of chloropreneconstitutes a definitely preferred modification of this invention.

It has been stated above that the stabilizing agents, disclosed herein,may be added to the polymer at any time after the polymerization processhas been carried to the point desired. Thus, they may be added to thepartially dried or dry polymer on the rubber mill. They may also beadded to a water dispersion of polymerized chloroprene beforecoagulation.

In fact, a very efficient procedure illustrated in Example 2 involvesthe addition of these agents to dispersions of polymerized chloropreneresulting from polymerizing chloroprene in emulsion, particularly in thepresence of modifying agents as disclosed in the copending applications,mentioned last above. Thus, a further preferred modification of thisinvention involves a variation of the modified polymerization processesdisclosed in the applications above referred to, which comprises addinga stabilizer, as disclosed herein, to the dispersion of heat curableplastic polymer obtained by polymerizing in the presence of a modifyingagent immediately after the polymerization has reached the desired pointand prior tocoagulation of the polymer.

It will be noted that as disclosed in this application and in anothercopending application of Walker, Serial No. 154,212, filed July 17,1937,

. 4 j many of the organic modifying agents disclosed in the,Starkweather and Collins application Serial Nd. 69,738, filed March 19,1938, are also-effective as stabilizing agents. It will also be f notedthat the long chain quanternary ammonium salts, disclosed as emulsifyingagents for use in the emulsion polymerization of chloroprene, etc., inthe presence of modifying agents are also eflective stabilizing agents,as disclosed in the copending application of Starkweather 1 and Collins,Serial No. 196,186, filed March 16,

1938. Certain of these modifying agents are,

j however, largely consumed during the polymerization under someconditions and the amounts of those emulsifying agents which are alsosta-' bilizing agents and which are present during the polymerizationare frequently insufllcient to produce the stabilizing eifect desired inthe finished 3 polymer or else'a portion of them is lost during somestage of the process. In general, therefore,

1 even though there are present during the formation of a particularpolymer some of the agents which are effective, both during thepolymerization and as stabilizers, it may be desirable to add additionalstabilizing agent to the polymer after the polymerization has reachedthe desired stage or at some later stage of the process in order toobtain the desired stabilizing effect in the final product.

i above, chloroprene is polymerized in an aqueous According to thepreferred process, outlined Q acid emulsion, in the presence of amodifying agent as disclosed in the copending applications, repeatedlyreferred to, until it has reached the desired stage of polymerizationunder the partic- 1 ular conditions used as indicated by previous tests,the nature of which is more fully set forth in the copendingapplications. At this point, the

polymerization is arrested by adding to the dispersion about 1%, basedon the polymer, of an antioxidant such as phenyl beta naphthylamine anda suflicient amount of an'aromatic mercaptan as disclosed herein, tobring the amount of stabilizer present up to about 2%, based on thepolymer, although the amount may be brought up to other amounts than 2%,as disclosed above.

replaced at a later stage of the process, prefer- }ably during the finalmilling step, that is, if the amount lost decreases the amount presentbelow the desired content.

This latter method of operation, i. e., stabilizaltion combined withmodified polymerization, is particularly advantageous, in that thestabilizer is much more readily incorporated while the polymer is stillin the emulsified state. Obviously, it is desirable to add thestabilizer as soon as is reasonably possible after the polymer hasreached the desired stage of polymerization, since \the function of thestabilizer is to retain the polymer in the state of plasticity in whichit exists at the end of the polymerization.

It is also possible to effect the addition of the stabilizing agentsdisclosed herein bya continuous process as well as a batch process. Thisis conveniently accomplished by bringing together a stream of adispersion of the polymer and a stream of a dispersion of thestabilizing agent in such a manner that they are thoroughly mixed; Theamount of stabilizer added is obviously readily controlled bycontrolling the rates of flow of the two streams. It is, of course,clear that the stream comprising the polymer as well as that comprisingthe stabilizer need not be a colloidal dispersion. Solutions may, forexample, be used. Obviously, however, whether one or both streams be asolution or a colloidal dispersion, means should be provided to mix themthoroughly, if thorough mixing does not otherwise take place.

Continuous emulsiflcation combined with continuous polymerization andalso combined with continuous addition of antioxidant is disclosed inthe copending applications, identified above,

which relate to modified polymerization and the combination ofcontinuous stabilization as described herein with continuous productionof polymer as described in those applications has been found to be verydesirable. Thus, a stream of a dispersion of a stabilizing agent asdisclosed herein is admitted into the pipe through which the dispersionof the polymer from the continuous polymerization is flowing, so thatthe two are thoroughly and continuously mixed. The amount of stabilizeradded is readily controlled by varying the rate at which the dispersionof stabilizer is admitted into contact with the polymer dispersion. Thiswill serve to illustrate the method. Obviously, continuous production ofstabilized polymer may be obtained without rigidly adhering to theparticular scheme described. The present invention therefore is notlimited to this exact method for continuous operation.

It is also possible to incorporate the antioxidant, for example,phenyl-beta-naphthylamine continuously in like manner, if an antioxidantis to be added. As disclosed above, the preferred procedure includes theaddition of an antioxidant. The antioxidant may be added continuously,either as a separate stream as disclosed in theapplications describingmodified polymerization or it may be included in the'same stream inwhich the stabilizer is included and, in fact, this latter variation hasbeen found to be very convenient. t

The uses of the heat curable plastic polymer stabilized according tothis invention are the same as those disclosed in the'copendingapplications, above referred to, which disclose the modifiedpolymerization and the advantages of the present invention are apparentfrom the above description. The compositions produced by the process ofthis invention are novel as'-. well as useful. These plastic polymercompositions possess the novel property of remaining in the'plasticstate for much longer periods than any previously known compositions ofsuch polymers, but

are substantially as readily converted under curing conditions to highlyelastic .products as they would be if the polymers had not been treatedwith a' stabilizer as described'herein. The stabilized polymers exhibita greatly reduced tendency to lose their plasticity on storage underordinary conditions for periods substantially in excess of three monthsor in the accelerated aging test at C. for periods in excess of 24hours.

It is apparent that many widely difi'erent embodiments of this inventionmay be made without departing from the spirit and scope thereof, and,therefore, it is not intended to be limited except as indicated in theappended claims.

--Cl, Br, I, NO2, and SOaM, in which M I claim: 1. Accompositioncomprising a heat curable plastic polymer of a compound of the generalformula CH2=CC=CH2 in which R is a member of the group consisting ofhydrogen and hydrocarbon radicals, and X is halogen having, in chemicalcombination therewith, an unpolymerizable, acid-stable organic modifyingagent which is capable of forming an addition product with compounds ofthe above general formula under conditions of polymerization ofcompounds of the above general formula, and also comprising a smallamount of a mercaptan of the general formula QS H in which the SH groupis attached directly to one of the carbon atoms in the Q group and inwhich Q is an organic group selected from the class consisting of arylgroups and aralkyl groups, which aryl groups and aralkyl groups consistof carbon and inorganic radicals selected from the class consisting ofH, Cl, Br, I, NO2, and SO3M, in which M is an inorganic radical of thegroup consisting of hydrogen and alkali metals.

2. A composition as' described in claim 1, further characterized in thatthe compound of the general formula cm=c-c=cm t I ischloro-2-butadiene-l,3.

3. The process which comprises incorporating into a heat curable plasticpolymer of a compound of the general formula CH2=CC=(II2 l l in which Xis halogen and R is a member of the group consisting of hydrogen or ahydrocarbon radical having, in chemical combination therewith, anunpolymerizable, acid-stable organic modifying agent which is capable offorming an addition product with compounds of the above general formulaunder conditions of polymerization of compounds of the above generalformula, a small amount of a mercaptan of the general formula Q-SH inwhich the SH group is attached directly to one of the carbon atoms inthe Q group and in which Q is an organic group selected from the classconsisting ofaryl groups and aralkyl groups, which aryl groups andaralkyl groups consist of carbon and inorganic radicals selected fromthe class consisting of H,

is an inorganic radical of the group consisting of hydrogen and alkalimetals.

4. The process of claim 3, further character ized in that the compoundof the general formula cHi=o-o=c11,

is chloro-2-butadiene-1,3.

5. The process which comprises dispersing about 100 parts by weight ofchloro-Z-butadiene- 1,3 in about 300 to 400 parts by weight of an acidaqueous medium and polymerizing the dispersed chloro-2-butadiene-1,3 inthe presence of 0.75'

is chloro-2-butadiene-l,3, the mercaptan is thiobeta-naphthol and thereis also present phenylbeta-naphthyl-amine.

7.- A composition as described in claim 1 further characterized in thatthe compound of thegeneral formula 'cin=( t-( :=cin

is chloro-2-butadiene-1,3, the mercaptan is nitro-thiophenol and thereis also present phenylbeta-naphthyl-amine.

8. The process which comprises dispersing chloro-2-butadiene-1,3 in anacid aqueous medium, polymerizing the dispersed chioro-2-butadiene-1,3in the presence of a small amount of thioglycolic acid and a smallamount of hydrogen sulfide, and adding a small amountofphenyl-beta-naphthylamine and a small amount of para-nitro-thiophenolto the resulting dispersion.

HERBERT W. WALKER.

